A service provider network includes one or more provider edge (PE) routers that extend attachment circuits to customer edge (CE) devices to provide services to customers of the service provider network. In some cases, the service provider network implements BGP/Multiprotocol Label Switching (BGP/MPLS) Internet Protocol (IP) Virtual Private Networks (VPNs) to segregate traffic for different customers by ensuring that routes from different VPNs remain distinct and separate, regardless of whether VPNs for respective customers have overlapping address spaces. For each VPN configured for the service provider network and in which a particular PE router participates, the PE router maintains a VPN Routing and Forwarding (VRF) instance. In general, each attachment circuit connecting a PE router and a CE device is associated with a VRF. For any given VPN, the PE router learns routes for the VPN, in some cases from the CE device, and installs the VPN routes to the corresponding VRF, which the PE router uses to forward traffic. In addition, the PE router distributes learned VPN routes to other PE routers of the service provider network (or to PE routers of one or more additional service provider networks) using BGP. BGP/MPLS IP VPNs are described in detail in Rosen & Rekhter, “BGP/MPLS IP Virtual Private Networks (VPNs),” Internet Engineering Task Force Network Working Group, Request for Comments 4364, February, 2006, which is incorporated herein by reference in its entirety (hereinafter “RFC 4364”).
In BGP/MPLS IP VPNs, PE routers use Route Target (RT) extended communities (“route targets”) to control the distribution of routes into VRFs. For a given collection of PE routers that peer using BGP, each PE router only stores VPN routes that are received and marked with route targets corresponding to VRFs that have local CE attachment circuits configured for the PE router. The PE router may discard all other VPN routes that it receives.
Because PE routers discard VPN routes that are not marked with a route target pertaining to a VRF of the PE router, techniques have been developed to constrain the distribution of VPN routes marked with a route target to those PE routers that will not discard the route target. For example, Marques et al., “Constrained Route Distribution for Border Gateway Protocol/MultiProtocol Label Switching (BGP/MPLS) Internet Protocol (IP) Virtual Private Networks (VPNs),” Internet Engineering Task Force Network Working Group, Request for Comments 4684, November, 2006 (hereinafter “RFC 4684”) defines Multi-Protocol BGP (MP-BGP) procedures that allow PE routers to exchange route target reachability information, which PE routers and route reflectors of a service provider network may use to build a route distribution graphs for route targets and thereby limit the propagation of VPN routes. RFC 4684 is incorporated herein by reference in its entirety.